1. Field of the Invention
This invention relates to an improved continuous process for the treatment of a rubber containing styrenic polymer composition.
More specifically, this invention relates to a two-step continuous treatment process, of a rubber containing styrenic polymer composition obtained by bulk polymerization or solution polymerization, for volatilizing and separating (hereinafter referred to simply as volatilize or volatilization) volatile components such as unreacted monomer, solvent, chain transfer agent, low molecular weight polymer and others from the polymer composition. In the first step, the volatilization of the volatile components takes place at a low temperature to permit a part of the unreacted monomer and solvent to remain, while in the second step the volatilization takes place under high vacuum, whereby the content of volatile components remaining in the final product is reduced to the lowest level without any deterioration in the physical properties of the final product and with low consumption of energy upon treatment. 2. Description of the Prior Art:
Conventionally, there have been disclosed a process for continuously removing the volatile components from the polymer composition, in which the composition is preheated or preheated while being foamed to be flashed into a volatilizer under reduced pressure (for example, Japanese Patent Publication No. 44323/85 and U.S. Pat. Nos. 3,928,300 and 3,966,538).
Recently, from the standpoint of food hygiene such as food packaging, and the like, it is desired to reduce the residual volatile components such as unreacted monomer, solvent and the like in the resin product to as low a level as possible and technical countermeasures therefor are an urgent problem.
In order to meet the above requirements, the prior art provides processes operating a preheater at a raised temperature and a volatilizer under high degree of vacuum. However, these processes have limits on the grounds below, and thus it is very difficult to reduce volatile components contained in the polymer composition below a predetermined level. That is, as the heating temperature in the preheater is raised, the remaining monomer polymerizes, thereby increasing the amount of low molecular weight polymers in the polymer composition, resulting in a reduced heat resistance of the molded product. Further, the elevated temperature of the polymer composition treated causes decomposition and color deterioration of the product and, in particular, in the case of rubber modified polymer composition, deterioration in the physical properties such as impact strength, due to cross linking of the rubber component. Moreover, to operate the volatilizer under an increased degree of vacuum causes the amount of gasified volatile components to increase, and thereby enhances the pressure drop during an evacuating pipe and a condenser in the volatile components recovery step. Thus, both the pipe and condenser having respective extremely large diameters are required, which cause extremely high costs. In certain cases, it was substantially difficult to provide an equipment operating under a certain high degree of vacuum.
To cope with the increased gas volume of the volatile components due to an elevated degree of vacuum in the volatilizer, there is disclosed a process for removing volatile components by using volatile component separation apparatuses in more than two steps (for example, U.S. Pat. No. 4,537,954).
However, according to the above processes, it was necessary to raise the temperature of a pretreated composition, prior to its being introduced into the final volatile component separation apparatus, to a certain high level in order to maintain the flowability of the treated mixture, because the composition contains only a small amount of volatile components, and has a high viscosity. In that event it was also necessary to add thereto a foaming agent in order to increase the surface area of the pretreated composition to conduct efficient foaming for volatization.
In manufacturing rubber containing styrenic polymers, it is required to maintain their impact resistance, tensile strength, heat resistance and color at respective high levels simultaneously with reduced contents of the volatile components therein. From industrial aspects, a simpler apparatus is desirable in removing volatile components.
As the prior art, there are disclosed processes for continuously removing the volatile substances in styrenic polymer compositions, in which the polymer compositions are treated at a temperature of 235.degree. C. or higher by using two step volatilization apparatus. U.S. Pat. Nos. 3,928,300 and 3,966,538 disclose a flashing room temperature of 235.degree. C., a degree of vacuum of 25 mmHg and a remaining styrene monomer content of 0.1%. Another process effects stirring in the first step volatilizer and/or the second step volatilizer (Japanese Patent Application Laid-Open Nos. 126412/84 and 126411/84). According to the process of U.S. Pat. Nos. 3,928,300 and 3,966,538, the content of the remaining styrenic monomer is not low satisfactorily, and the treating temperature of the polymer composition is so high that deterioration of a rubber component is promoted when a rubber-modified styrenic polymer composition is treated. Thus, the processes by the U.S. patents are unsatisfactory as a treatment process of rubber-modified styrenic polymer composition. The processes disclosed in Japanese Patent Application Laid-Open Nos. 126412/84 and 126411/84 necessitate stirring in the volatilization step, and the process disclosed in Japanese Patent Application Laid-Open No. 126411/84 necessitates addition of an antioxidant.